Colored iron-stained coal and a method of coloring



"United States Patent ice 3,019,121 COLORED IRON-STAINED COAL AND A METHOD OF COLORING Richard L. Newman, Flemington, N..l'., assignor to American Cyanamid Company, New York, N.Y., a

corporation of Maine No Drawing. Filed Aug. 4, 1958, Ser. No. 753,121 14 Claims. (Cl. 106-286) This invention relates to the coloring of coal and more particularly to an improved process of coloring coal using pigment compositions with reagents effecting color formation in situ.

For many years it has been customary to trademark coal, especially anthracite coal, by coloring it. The largest use has involved a blue coloration of the coal. The first method used was to apply to the wet coal a dispersion of ferric ferricyanide followed by a brief exposure to a uni-directional electric current of suitable polarity which, in conjunction with the reducing nature of the coal surface which acted at least in part as one of the electrodes, reduced one of the constituents of the ferric ferricyanide molecule to the ferrous state of oxidation, thus transforming the slightly brown ferric ferricyanide solution into one of the deep iron blues, The thin non-uniform layer of iron blue produced on the coal surface resulted in an effect of striking beauty as well as a very clearly recognizable blue coloration. The process, though used for a short time with excellent technical results, proved to be too expensive for practical use because of the low unit cost of coal and even with improvements, such as the application of the color to freshly fractured coal, was not able to survive because of cost.

The next attempts to produce a practical, cheap blue coloration of coal involved the use of meta-stable dispersions of ultramarine products, for example Ultramarine associated with higher fatty acids or with relatively waterinsoluble salts thereof. The ultramarine dispersion was applied directly to the wet coal, the dispersion broke almost immediately and left patches of ultramarine on the coal with adequate adherence, though not of quite the degree of permanent adherence which was achieved by the reduced ferric ferricyanide method. The cost of coloration with ultramarine was but a small fraction of the cost of the reduced ferric ferricyanide coloring method and completely superseded it. For more than a decade this ultramarine process has been the standard method of coloring coal blue.

In spite of the great and continuing practical success of the ultrarnarine coal coloring process, certain drawbacks arose. Many seams of coal, particularly anthracite coal, or in some cases portions of seams, contained deposits of iron compounds in irregular patches. These iron com ounds are of predominantly yellow, reddish yellow to yellowish red color, some being duller than others and approaching more a brownish yellow. The iron stains make the coal appear dirty and unattractive; and, unfortunately since they are almost a perfect complementary color for the reddish blue of the ultramarine patches, the latter greatly accentuates by contrast the iron stains so that they are more noticeable on ultramarine colored coal than on uncolored coal.

As the blueing of coal is for the purpose of rendering its appearance distinctive and more attractive, the ugly looking iron stains have presented an increasingly serious problem in consumer resistance. Although the amount of iron compounds in the stain is infinitesimal compared to the weight of the coal and in no way seriously interferes with its burning characteristics, coal dealers and consumers are not technically trained and wrongly interpret iron-stained blued coal as being an inferior product.

3,019,121 Patented Jar 1.30, 1962 It is with the solution of the problem of iron stains that the present invention deals.

Essentially, the present invention solves the iron stain problem by transforming the iron stains, or the surfaces thereof, into an iron blue, a complex ferric ferrocyanide. The method by which this transformation is effected is, however, a peculiar one. It has been found that satisfactory elimination of iron stains can be effected by treating the coal with a solution of a soluble ferrocyanide, such as sodium, potassium, or ammonium ferrocyanide. The mechanism of the reaction is not known, and it appears at first glance to violate the ordinary laws of physical chemistry. There is obviously some peculiar result; but i the precise mechanism is not known, and the present invention is not limited to any theory.

The unexpectedness, and in fact the apparent impossibility, of the result can be readily appreciated from the following conclusions. The iron stains are of indeterminate composition. For the most part they are ferric oxides, hydroxides, hydrated oxides, and some complex iron salts. Predominantly, they are hydrated oxides because they are usually of the more yellowish tinge rather than the deeper red of pure ferric oxide.

Not all of the constituents of the iron stains are completely insoluble in water; but, in terms of a very short time, of a few seconds, they may be considered as insoluble. Thus, if a piece of coal is subjected to a stream of cold water for hours, a negligible amount of iron stain dissolves. In fact, the customary treatment of mined coal in the coal breaker involves continuous contact with water for a relatively extensive period of time. In a few seconds, therefore, there is no substantial solubility in any iron stain; yet the coloring of coal by the present invention results in the formation of the iron blue on the stain in a matter of a few seconds.

Ordinary laws of physical chemistry give quantitative evidence that the reaction between a dissolved reagent and a substantially insoluble reagent proceeds with extreme slowness. Rapid reaction which can be measured in reaction times of seconds, or even a minute or so, requires that the reagents be in solution, or at least in such fine dispersion as in a colloidal solution, so that the surface area of reagent particles is enormous compared to their volume. Under the ordinary considerations of physical chemistry it would be expected that in a few seconds of contact there would be substantially no reaction between the relatively insoluble ferric compounds in the iron stain and the ferrocyanide in the ultramarine emulsion; yet it has been found that in a few seconds contact, the whole surface of the stain reacts with the ferrocyanide and there is a light blue color over the whole of the stain. The resulting coal is of excellent appearance, does not show ugly iron stains, and is more acceptable to consumers. What the unknown mechanism is that permits a substantially instantaneous reaction with an almost completely insoluble reagent is not known. It cannot well. be an ordinary chemical reaction in aqueous solution because in such a case it would not be obeying the ordinary laws of physical chemistry. Some other mechanism is obviously involved. What it may be is not known and, in fact, there is not even sufficient evidence to advance any tentative speculation.

When the stained coal is treated with the ferrocyanide solution only, the surface of the yellow stains becomes light blue and the coal from a short distance appears much blacker. However, the depth of the blue coloring is not sufiicient to make the coal appear blue and, therefore, where for trademark or other purposes it is desired to have a definitely blue coal, the ferrocyanide treatment is combined with a pigment dispersion, such as a dispersion of ultramarine or other blue pigment.

This combination constitutes a preferred embodiment of the invention where it is ,desired to have the coal colored a definite color and not merely to disguise the iron stains.

The present invention has an advantage that it does not depart substantially, as far-as the pigment portion of the treating dispersion is concerned, from conventional practice. The metastable emulsions used are the same as those which have been in standard use for many years. The mechanism of emulsion breaking and ultramarine aflixing to the coal is thus not afiected adversely in any way.

It is a further advantage that the amount of alkali metal ferrocyanide is in no way critical. A very small amount may be used, although, of course, there is a practical lower limit. Thus, results begin to fall off when the amount of alkali metal ferrocyanide falls below 80 parts per million based on the coal. A usable efiect is still obtained with as little as 40 parts, but the covering of the iron stains is not quite complete. The 40 parts per million may therefore be considered as the practical lower limit. In general it is desirable to use a larger amount of sodium ferrocyanide, as it is quite a cheap chemical and less precise control of the process becomes necessary; for example, very satisfactory practical results are obtained with from l60-300 parts per million of the ferrocyanide. There does not appear to be any upper limit, as excellent results are obtained with as much as 1200 parts per million. Of course, a practical operator will not use enormous excesses and thus add to his cost if no beneficial result is obtained, and so what upper limit there is to the amount of ferrocyanide used is a purely practical and economical one and is not related to the technical operativeness of the present invention which is therefore not limited to any upper limit of ferrocyanide.

It is a further advantage that the point of introduction of the soluble ferrocyanide is in no way critical. It may be added to the colored dispersion and form a part of the aqueous phase thereof. The addition may be at the time of coal treatment or, of course, the coal may be treated in two steps, with the colored emulsion and with the sodium ferrocyanide. The order of the treatment is immaterial. For practical operating purposes it is preferred to use a single-step process in which the alkali metal ferrocyanide is incorporated with the colored dispersion before it is applied to the coal.

The above description of the present invention and of the problems solved thereby has been made in conjunction with coloring of coal blue with ultramarine, as this is by far the most common color applied to coal and, as it happens, is of such a nature that the iron stains become more noticeable by contrast. The present invention, however, is not limited to processes using a colored dispersion and is equally effective when other pigments are used, as they have been in the past, for coal coloring; such as for example, aluminum powder, phthalocyanine blue, phthalocyanine green, the red pigment obtained by coupling beta-naphthol to diazotized Tobias acid, which is usually referred to as Lithol Red and which is customarily sold as the barium lake. Other pigments may be used, such as lead chromate, p-re-formed iron blues, and mixtures.

As has been pointed out above, the present invention does not make any change in the pigmented dispersions themselves, except for the addition of the ferrocyanide.

Any of the conventional dispersions may be employed, such as oil-in-water emulsions which are described in US. Patent 2,621,115. A typical dispersion will be illustrated in the examples, it being understood that the pigmented dispersion is not in any way changed by the application of the process of the present invention. Other dispersions of water-repellent ultramarine are described in U.S. Patents 2,323,748 and 2,323,749.

The invention will be described in greater detail in conjunction with the following specific examples in which the parts are by weight unless otherwise specified.

Example 1 Two parts by weight of an emulsifying agent produced by the reaction of para-octyiphenol with ten moles of ethylene oxide was dissolved in 3200 parts of water. Thereupon, 116 parts of a high-boiling cyclic hydrocarbon oil are added with stirring. 6400 parts of water, 200 parts of dry ultramarine blue are then stirred in and finally 180 parts of sodium ferrocyanide are added and dissolved.

The above emulsion was used for coloring iron-stained coal by two methods. In the first the coal was passed through the emulsion and in the second the emulsion was sprayed on the coal, the excess being permitted to drain off. In each case the amount of emulsion used was sufficient for application to 600,000 pounds of coal.

Both methods of treatment gave the same result and in each case the yellowish iron stains were transformed into a greenish iron blue.

Example 2 The procedure of Example 1 was repeated increasing the amount of sodium ferrocyanide to 720 parts. The results were identical.

Example 3 The procedure of Example 1 was repeated reducing the amount of sodium ferrocyanide to parts. No deterioration in result was noted.

Example 4 The procedure of Example 1 was repeated using 50 parts of sodium ferrocyanide. The results were substantially the same as in Example 1, the intensity of the blue decreasing slightly.

Example 5 The procedure of Example 1 was repeated using 25 parts of sodium ferrocyanide. Coloring of iron stain took place but was substantially less than in Example 4.

Example 6 The procedure of Example 1 was followed, the sodium ferrocyanide being reduced to 15 parts. The coloring of the iron stain was very slight and this amount of sodium ferrocyanide was below that which gives any useful results.

Example 7 The procedure of Example 1 was repeated leaving out the ferrocyanide entirely. The ultramarine blue colored the coal in spots but the iron stains were substantially untouched and the general appearance of the coal was worse than untreated coal since the blue of the patches of ultramarine accentuated the yellow color of the stain.

Example 8 The procedure of Example 1 was repeated replacing the emulsion with a dispersion of ultramarine coated with aluminum oleate as described in Example 1 of Patent 2,323,748. The iron stains were all colored blue and a satisfactory blue coloring from the ultramarine resulted, showing a slightly lower adhesion than the emulsion of Example 1 and, of course, not possessing any resistance to dusting which is imparted by the hydrocarbon oil in the emulsion of Example 1.

Example 9 The procedure of Example 1 was repeated replacing the sodium ferrocyanide with an equivalent amount of potassium ferrocyanide. The results were identical with that of Example 1, showing that the process is substantially independent of the chemical nature of the cation of the ferrocyanide.

Example The procedure of Example 1 was repeated but instead of adding the sodium ferrocyanide after the emulsion was formed it was added to the water solution of the emulsifying agent before the emulsification took place. The emul sion formed was the same as in Example 1 and the coal showed the same satisfactory blue color of the iron stains.

Example I] The following comparative example was carried out by treating iron-stained coal with 1.8% solution of potassium ferricyanide and sodium ferrocyanide, respectively. These solutions were then used in treating iron-stained coal by dipping. The sodium ferrocyanide turned the iron stain blue but the potassium ferricyanide produced no blue coloration.

Example 12 Example 1 was repeated replacing the sodium ferrocyanide with the same weight of potassium ferricyanide. As in Example 11, no blue coloration of the iron stains on the coal resulted.

Example 13 The procedure of Example 1 was repeated replacing the ultramarine by an equivalent amount of the red pigment Lithol Red. The coal was colored with red spots of the red pigment and the yellow iron stains were trans formed into blue as in the case of Example 1.

Example 14 The procedure of Example 1 was repeated using aluminum powder in place of the ultramarine. Again the coal was colored with patches of aluminum color and the iron stains were colored blue.

Example 15 The procedure of Example 1 was repeated using 200 parts of ferric ferrocyanide instead of 200 parts of ultramarine blue. The shade of the colored coal is greener than the product of Example 1 and the iron stains are the same color.

Example 16 Example 15 was repeated substituting 97.5 parts of lead chromate yellow for 97.5 parts of the iron blue. The coal is colored a light chrome green and the iron stains are colored blue.

Example 1 7 Iron-stained coal was subjected to a two-step process, the first step being with a .25% solution of sodium ferrocyanide and the second step with a pigmented oil-in-water emulsion as described in Example 1 but without any ferrocyanide present. The results were the same as in Example 1, that is to say the coal was colored in blue patches with the ultramarine and the iron stains were transformed into blue color.

Example 18 prises subjecting the coal surface to the action of an aqueous dispersion of a pigment and a water soluble ferrocyanide, the said ferrocyanide being used in amounts no less than 40 parts per million based on the coal, whereby portions of the coal are colored by the pigmented dispersion and the surface of the iron stains is transformed into a blue color.

2. A process for treating iron-stained coal which comprises contacting the coal surface with a dispersion of ultramarine and a dilute aqueous solution of a soluble ferrocyanide, the said ferrocyanide being used in amounts no less than 40 parts per million based on the coal used.

3. A process according to claim 2 in which the soluble ferrocyanide is sodium ferrocyanide.

4. A process according to claim 3 in which the dispersion is an oil-in-water emulsion.

5. A process according to claim 1 in which the coal is sprayed.

6. A process according to claim 1 in which the coal is dipped.

7. A process according to claim 2 in which the ultramarine dispersion contains the soluble ferrocyanide in solution and the coal is contacted therewith in a single step.

8. A process according to claim 2 in which the soluble ferrocyanide is in a separate solution from the dispersion of the ultramarine.

9. A process of treating iron-stained coal which com prises subjecting the coal to the action of an aqueous solution of a soluble ferrocyanide, the said ferrocyanide being used in amounts no less than 40 parts per million based on the coal used, whereby the surface of the iron stains istransformed into a blue color.

10. Colored iron-stained coal, the surface of which is colored with discrete areas of pigment, the said pigment being selected from the group consisting of ultramarine, Lithol Red, aluminum powder, lead chromate, and ferric ferrocyanide and the surface of the iron stains is transformed into ferric ferrocyanide.

11. Colored iron-stained coal, the surface of which is colored with discrete areas of ultramarine and the surface of the iron stains is transformed into ferric ferrocyanide.

12. Iron-stained coal of uniform enhanced black surface color in which the surface of the iron stains is transformed into ferric ferrocyanide.

13. An oil in water emulsion for coloring iron-stained coal consisting essentially of an emulsifying agent, a pigment, a hydrocarbon oil, a water soluble ferrocyanide salt and a major amount of water, the ferrocyanide salt constituting at least about 0.25% by weight of the entire emulsion.

14. The com-position of claim 13 wherein the pigment is ultramarine.

References Cited in the file of this patent UNITED STATES PATENTS 2,870 Ferris Dec. 5 1842 108,615 McIlvaine Oct. 25, 1870 2,129,901 Glinsman Sept. 13, 1938 2,129,902 Barsky et a1. Sept. 13, 1938 2,312,898 Ham et a1. Mar. 2, 1943 2,323,748 Dieterle July 6, 1943 2,323,749 Dieterle July 6, 1943 

1. A PROCESS OF TREATING IRON-STAINED COAL WHICH COMPRISES SUBJECTING THE COAL SURFACE TO THE ACTION OF AN AQUEOUS DISPERSION OF A PIGMENT ANDA WATER SOLUBLE FERROCYANIDE, THE SAID FERROCYANIDE BEING USED IN AMOUNTS NO LESS THAN 40 PARTS PER MILLION BASED ON THE COAL, WHEREBY PORTIONS OF THE COAL ARE COLORED BY THE PIGMENTED DISPERSION AND THE SURFACE OF THE IRON STAINS IS TRANSFORED INTO A BLUE COLOR. 